1. Field of the Invention
The invention concerns a process for producing stainless steels, especially high-grade steels containing chromium and chromium-nickel.
2. Description of the Related Art
Multistage processes in melting equipment that comprises at least two vessels are well known for the production of high-grade steels that contain chromium or chromium-nickel. Depending on the particular process technology, decarbonization is carried out down to carbon contents of less than 0.3%. There is always a high energy requirement, and temperature losses are unavoidable.
A process of this type is known from DE 196 21 143. The process described there is carried out in melting equipment that comprises at least two vessels. The two vessels are operated in parallel in such a way that alternately either electrodes can be used for melting the charge or blowing lances can be used for top blowing and/or injection of oxygen and oxygen mixtures. The vessels are thus used first as melting equipment and then as decarbonization equipment. After the blowing, the slag is reduced with reducing agents, such as ferrosilicon, aluminum, or secondary aluminum, with the addition of fluxes, such as lime and fluorite, for the recovery of oxidized chromium, and the slag is then tapped. The goal of the invention is to make a process of this type more economical.
The essence of the invention is the reversible treatment of unreduced converter slag in electric-arc furnace operation. In contrast to the well-known process, in which the reduction of the high-chromium slag and thus the recovery of the metallic chromium is carried out in a process step that follows, and is separate from, the melting and oxygen blowing, in this case the reduction is carried out simultaneously with a renewed melting operation of a new charge with retention of the slag of the preceding air-refining process in the vessel. In this way, a process step, namely, the subsequent reduction of the slag, is saved, and the chromium-containing slag is not removed from the system. All together, this makes the process simpler and more economical.